The Open Web Application Security Project has released the 2004 Top Ten Web Application Vulnerabilities.
http://www.owasp.org/documentation/topten
A good read. last year, I just started at a new job when I saw this report. The app I was working on was originally done by someone else and was guilty of 5 or 6 of those vulnerabilities.
something a lot of newbies should read before tackling those e-commerce sites.